${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ hyperfine structure in an optically induced electron-spin-resonance spectrum has been detected in hydrogenated amorphous silicon using a pulsed technique. The optically induced ESR signal is accompanied by an increase in the subbandgap optical absorption. These observations suggest that the optically induced ESR signals originate from a localized state similar to the singly occupied dangling-bond state observed in dark, which is phenomenologically similar to the defect properties in chalcogenide glasses.

We have directly observed hot-electron energy loss through the emission of phonon-plasmon coupled modes in n-type Si-doped GaAs. The self-compensation of silicon in GaAs was exploited to allow observation of hot-electron recombination with neutral Si acceptors in a high background concentration of free electrons. We have obtained hot-luminescence data which exhibit a peak due to the initial unrelaxed hot electrons followed by a peak lower in energy by the ${\mathit{L}}^{+}$ coupled-mode energy. The lower-energy peak corresponds to hot-electron relaxation via emission of a phonon-plasmon coupled mode. The observed peak agrees well with energy-loss calculations.

Coherent LO phonons are excited at the surface of bulk GaAs with femtosecond laser pulses. They are observed for the first time through electro-optic modulations of the transient reflectivity. The corresponding signal oscillations are superimposed upon an additional longitudinal polarization feature which decays exponentially and whose rise time is sufficiently short to act as a driving force for the coherent phonons. The components of the coherent phonon states dephase relative to each other in the order of picoseconds depending on the density of optically excited electron-hole pairs.

A self-consistent Schrödinger equation is derived to describe the electrostatic potential distribution of a two-dimensional electronic system confined in a narrow wire in a weak magnetic field using the Hartree approximation. Solving the equation numerically in the lowest lateral mode limit, the electric potential and current distributions across the wire are calculated. It is shown that the Hall voltage defined as the electrostatic potential difference between the two edges of the wire is not quenched but retains the classical linear dependence on the magnetic field. The possibility of a novel weakly coupled voltage measurement is also proposed.

Narrow channels interrupted by two controlled potential barriers and having a tunable electron density were made in GaAs, and their conductance was measured at low temperatures. Reproducible and accurately periodic oscillations of the conductance with changing density are found to correspond to the sequential addition of single electrons to the segment of the channel between the barriers. Detailed examination of the line shape of the conductance versus density provides a new insight into the transport mechanism.

For coupled double-quantum-well systems in which tunneling is important, the symmetric to antisymmetric energy gap leads to a quantum Hall effect. In this Letter we show that interaction effects in strong magnetic fields can destroy this gap, and present a theory which predicts the occurrence or nonoccurrence of a quantum Hall effect.

Quantum potential scattering from abrupt scatterers is studied nonperturbatively in the quantum Hall regime. Complex scattering dynamics including sharp resonances are observed for the first time in this regime. Inter-Landau-level tunneling is strongly enhanced by extended scatterers. The calculated threshold field is in agreement with breakdown experiments. An exact transfer-matrix method and a quasiclassical Wigner-function method are used to extract both amplitudes and phases of the scattering matrix.

A scanning tunneling microscope is used to locally deposit arrays of nanometer-scale magnets directly within the superconducting planar input coil of an integrated dc SQUID microsusceptometer. Low-temperature frequency-dependent magnetic-susceptibility measurements reveal a narrow resonance which grows and becomes independent of decreasing temperature. Studies as a function of magnet volume, spacing, and applied field are compared to recent predictions for magnetic macroscopic quantum tunneling in small particles.

For a ferromagnet with exchange, uniaxial anisotropy, demagnetizing field, and an external magnetic field directed oppositely to the magnetization, the Landau-Lifshitz equations have time-independent, planar, local solutions of the Bloch and Néel type. These solutions are unstable. Analogous to a water droplet in supersaturated vapor for which a radius of unstable equilibrium exists, the solution for a given magnetic field is the configuration of smallest energy through which a magnetization reversal can nucleate.

The first observation of a pressure-induced metallization accompanied by the spontaneous collapse of magnetic ordering is reported. This isostructural concurrence is observed in the layered antiferromagnetic insulator ${\mathrm{NiI}}_2$. Electrical transport, x-ray diffraction, and ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ Mössbauer studies were carried out in diamond-anvil cells. The evidence points to a mechanism by which the originally filled 5p-iodide band shifts and overlaps with the partially filled nickel-3d states. The formation of a hybridized conduction band at P>19 GPa results in the onset of a diamagnetic metallic ground state.

We have studied flux melting near the ${\mathit{H}}_{\mathit{c}1}$ line in T-Ba-Cu-O using a variational Monte Carlo method. The existence of a flux-liquid phase has been observed and the transition line between flux liquid and flux solid in the H-T phase diagram has been determined. The width of the liquid phase increases with temperature as ${\mathit{T}}^2$ at low temperatures, as predicted by Nelson, and faster than ${\mathit{T}}^2$ at higher temperatures, eventually crossing over to the high-field melting curve.

The linear conductance background observed in tunneling into the high-${\mathit{T}}_{\mathit{c}}$ superconductors has often been interpreted as a density-of-states effect. Here we argue that it may be due instead to strong inelastic electron tunneling from a broad, flat continuum of states. We provide a specific model for such a continuum of states in the high-${\mathit{T}}_{\mathit{c}}$ superconductors based upon a form for the spin-fluctuation spectral weight which fits NMR data, and compare the predictions of this model with experiment.

Using polarized reflectivity measurements of single-domain crystals, we are able to distinguish chain and plane contributions to the infrared conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. A substantial chain contribution to σ(ω) persisting to low frequency and temperature is observed. For the intrinsic conductivity of the ${\mathrm{CuO}}_2$ planes a superconducting energy gap of 500 ${\mathrm{cm}}^{\mathrm{-}1}$ (2Δ/${\mathit{kT}}_{\mathit{c}}$≃8) is evident in the infrared data, while the normal-state conductivity drops much more slowly with ω than the ordinary Drude form, and can be described in terms of a scattering rate ħ/${\mathrm{\tau }}^{\mathrm{*}}$∼kT+ħω at low frequency. The former result (2Δ/${\mathit{kT}}_{\mathit{c}}$≃8) suggests suppression of ${\mathit{T}}_{\mathit{c}}$; the latter, that ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ is not an ordinary Fermi liquid.

A numerical study of a set of random matrices which interpolate Poisson and Gaussian orthogonal ensembles is reported. The result indicates that the transition from Poisson to Wigner distributions for the nearest-level spacing does not depend on the details of the random-matrix parametrization and is essentially governed by a single parameter. Brody’s one-parameter interpolation formula is found to describe the transition rather well.

Evidence is presented for the existence of a strange nonchaotic attractor in a two-frequency quasiperiodically driven, buckled, magnetoelastic ribbon experiment. Scaling behavior in the Fourier amplitude spectrum is observed in agreement with predicted scaling behavior for strange nonchaotic attractors. Dimension measurements also support the existence of a strange nonchaotic attractor.

We report here the most precise constraint to date on the spectrum of the cosmic background radiation (CBR), obtained from measurements made with a liquid-helium-cooled spectrometer carried above the atmosphere on a rocket. The spectrum is very well fitted by a Planck function of temperature T=2.736 K. The scatter of the equivalent temperature in the band 3–16 ${\mathrm{cm}}^{\mathrm{-}1}$ is ±10 mK, about 1/3% of the mean whereas the estimated overall accuracy of the mean temperature is ±17 mK. These results are inconsistent with a previously reported excess IR intensity in the CBR but are in good agreement with COBE results.

We show how to extend the theory of chiral scalars described by the Floreanini-Jackiw Lagrangian to include interactions and maintain a Lorentz-type symmetry. We also show how, at the Lagrangian level, the sum of two such left and right chiral scalars reproduces the usual scalar theory.

By using a dynamical model of chiral-symmetry breaking, we demonstrate that the chiral perturbation expansion around the ${\mathrm{SU}}_{\mathit{f}}$(3)-symmetric vacuum does not converge in the large-${\mathit{N}}_{\mathit{c}}$ limit. The convergence radius of the expansion in the strange-quark mass ${\mathit{m}}_{\mathit{s}}$ is shown to be 30–50 MeV, which means that higher-order terms become larger as one increases the order of the expansion in the physical region (${\mathit{m}}_{\mathit{s}}$∼150 MeV). The first-order mass formula for the Nambu-Goldstone bosons turns out to be accidentally valid in the physical region.

Inelastic electron scattering is used to identify nine stretched isovector [${\mathit{T}}_{>}$ (${\mathit{T}}_0$→${\mathit{T}}_0$+1) M6 transitions in ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ (${\mathit{T}}_0$=0) which exhaust (77±17)% of the extreme single-particle-hole-model sum rule. This is the first time that several prominent ${\mathit{T}}_{>}$ electromagnetic stretched transitions have been observed in a self-conjugate nucleus and the first time that such a large fraction of the sum rule has been observed. These effects are primarily due to the presence of spectator nucleons in the 2${\mathit{s}}_{1/2}$ orbit, as substantiated by shell-model calculations.

Cross sections at 0° for the pure Fermi ($0^{+}$, 2.31, MeV) and Gamow-Teller ($1^{+}$, 3.95 MeV) states in the ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$(p,n${)}^{14}$N reaction have been measured at ${\mathit{E}}_{\mathit{p}}$=494, 644, and 795 MeV using the LAMPF neutron time-of-flight facility with a new 617-m neutron flight path. The measured cross sections per unit transition strength, σ^, provide a measure of the isovector spin-flip and non-spin-flip central components of the effective nucleon-nucleon interaction. Cross sections and the ratio ${\mathit{R}}^2$=σ${\mathrm{^}}_{\mathrm{GT}}$/σ${\mathrm{^}}_{\mathrm{F}}$ are compared to lower-energy measurements and to calculations using a free NN t matrix.